Quadrupolar nuclei have nuclear spin quantum numbers greater than one half. Common nuclei of this Kind include .sup.7 Li, .sup.9 Be, .sup.11 B, .sup.14 N, .sup.23 Na, .sup.27 Al, .sup.35 Cl, .sup.39 K, .sup.55 Mn, .sup.59 Co, .sup.75 As, .sup.79 Br, .sup.81 Br, .sup.127 I, .sup.197 Au, .sup.209 Bi and many others--this list should not be interpreted in any limiting way, it is given simply to note the potential range of substances which might be detectable by this technique.
In the sub-molecular environment of compounds or crystals, the nature and disposition of the adjacent electrons and atomic nuclei produce electric field gradients which modify the energy levels of any quadrupolar nuclei, and hence give rise to transition frequencies, to such an extent that measurements of these transition frequencies and/or relaxation time constants can indicate not merely the nuclei which are present but also their chemical environment.
NQR measurements have the great advantage that they do not require the sample to be placed in a strong magnetic field, and therefore do not require the large, expensive and sample-size limiting magnet structures which are needed for nuclear magnetic resonance (NMR) measurements. It should be noted that the rules governing NQR effects are so significantly different from the rules controlling NMR effects that practices and principles established in NMR studies do not necessarily apply in NQR work.
United Kingdom Patent Application No. GB-A-2159626 discloses a nuclear magnetic resonance spectrometer comprising: a means for producing a static magnetic field; a plurality of transmitter/receiver coils placed in the magnetic field; a means for repeatedly producing an RF pulse containing the resonance of a nuclide to be observed; a switching means for allowing the RF pulses to be successively supplied to the coils and applied to a sample under examination; a receiver circuit for picking up the resonance signals detected by the coils after the application of the RF pulses; and means for separately storing the data about the picked up resonance signals for the different coils. Means is also provided for moving the sample relative to the coils, or vice versa.
U.S. Pat. No. 4,887,034 describes methods by which the presence of various drugs or explosives may be detected indirectly through the interaction of NQR and NMR effects, but these methods require a strong magnetic field and the presence of NMR nuclei as well as quadrupolar nuclei in the substances to be detected. The strong magnetic field would require relatively large magnets with the added complication that the magnetic field has to be varied between strong and weaker values.